The present invention relates generally to safety devices for detecting over-power conditions in motor driven systems, and more particularly relates to such safety devices and methods for use in irrigation systems utilizing multi-phase induction motors.
Many systems utilize heavy duty motors. The motors may be utilized for running sub-systems or for transporting the system itself. For example, grain drying machinery and oil field drilling equipment are systems of the first type, and center pivot irrigation systems are systems of the latter type. In both types of systems requiring the heavy duty motors, occurrences of large amounts of current for operating the motors is unavoidable, necessitating utmost safety precautions. For example, during start-up of the motors, extraordinarily high magnitudes of current are encountered. During normal operation of the motors, lesser amounts of current drawn than during start-up, but even such lesser amounts may be hazardous. This is especially so if a malfunction should occur, such as a short circuit in the system. Such a short circuit usually results in high values of current being directed into the chassis of the system.
Short circuit malfunctions have long been recognized as hazardous, and attempts have beem made to monitor such malfunctions using ground fault indicators (GFI). GFI's have conventionally been designed to monitor current going into the system on a given line versus current coming out of the system on the line. The value of the measured differential is indicative of a potentially dangerous ground fault. Although such current measuring systems have generally been satisfactory for detecting malfunctions in single phase motor systems, multi-phase systems have posed problems because of the nature of three phase systems and the functional relationships among the phases.
In multi-phase systems, for example, a fault usually creates an imbalance of voltages between the phases. There is no simple current path between the phases for monitoring according to conventional GFI technique. Further, the voltage imbalance tends to produce a voltage differential between the ground wire and chassis and may provide a hazardous voltage without the attendant current drain. Accordingly, current sensing schemes are inadequate to detect all fault situations in multi-phase systems.
Further, because of the high inrush of current upon starting, such motor driven systems are unsuited for simple fusing techniques. For example, because of the initial high inrush of current, the fuse either must be of a sufficiently high value to withstand the high inrush, and thereby allowing a lesser standard of safety, or it must be protected during starting of the motors. However, even when protected, the value selected for the fuse is usually of a magnitude which eliminates nuisance shutdowns. For example, in center pivot irrigation systems, the heavy duty motors which rotate the irrigation system encounter differing terrain conditions, calling for differing amounts of torque. This varying torque characteristic causes a corresponding variance in the amount of current drawn by the motors. Unless the value of the fuse is selected at a sufficiently high value to account for extreme operating current requirements, the system often becomes unacceptably shut down during normal operations. On the other hand, if the fuse value is selected to avoid such nuisance shutdowns, the lesser standard of protection is provided.